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Abstract:

A liquid crystal optical device that includes a first substrate layer
that is substantially flat and a second substrate layer that is
substantially flat and parallel to the first substrate layer. The liquid
crystal optical device further includes a layer of cholesteric liquid
crystal disposed between the first substrate layer and the second
substrate layer, where the layer of cholesteric liquid crystal is
arranged in domains, each domain having a helical axis, wherein the
helical axes of the domains have a plurality of orientations relative to
an orientation of the first and second substrate layers, and where a
wavefront of a light wave having a wavelength within a range of
wavelengths changes after reflecting from the layer of cholesteric liquid
crystal.

Claims:

1. A liquid crystal optical device, comprising: a first substrate layer
that is substantially flat; a second substrate layer that is
substantially flat and parallel to the first substrate layer; and a layer
of cholesteric liquid crystal disposed between the first substrate layer
and the second substrate layer, wherein the layer of cholesteric liquid
crystal is arranged in domains, each domain having a helical axis,
wherein the helical axes of the domains have a plurality of orientations
relative to an orientation of the first and second substrate layers, and
wherein a wavefront of a light wave having a wavelength within a range of
wavelengths changes after reflecting from the layer of cholesteric liquid
crystal.

2. The liquid crystal optical device according to claim 1, wherein the
layer of cholesteric liquid crystal acts as a positive lens or a negative
lens.

3. The liquid crystal optical device according to claim 1, wherein, for a
first domain of cholesteric liquid crystal, a pitch of the cholesteric
liquid crystal varies along the helical axis of the first domain.

4. The liquid crystal optical device according to claim 1, wherein the
layer of the cholesteric liquid crystal is substantially flat.

6. The liquid crystal optical device according to claim 1, further
comprising: a first electrode coupled to the first substrate layer; and a
second electrode coupled to the second substrate layer, wherein the layer
of cholesteric liquid crystal is disposed between the first electrode and
the second electrode.

7. The liquid crystal optical device according to claim 1, wherein at
least one of the first electrode and the second electrode is transparent,
and the layer of cholesteric liquid crystal changes state based on a
voltage applied to the first electrode and the second electrode.

8. The liquid crystal optical device according to claim 1, further
comprising at least one additional layer of cholesteric liquid crystal.

9. The liquid crystal optical device according to claim 8, wherein the
liquid crystal optical device comprises two layers of cholesteric liquid
crystal, wherein the two layers of cholesteric liquid crystal have the
same handedness or opposite handedness.

10. The liquid crystal optical device according to claim 8, further
comprising a layer of birefringent material disposed between each layer
of cholesteric liquid crystal.

11. The liquid crystal optical device according to claim 10, wherein at
least one layer of birefringent material has a phase retardation it for
wavelengths within the range of wavelengths.

12. The liquid crystal optical device according to claim 11, wherein the
phase retardation is based on an electric field applied to the
birefringent material.

13. The liquid crystal optical device according to claim 1, wherein the
range of wavelengths comprises a range of reflection in accordance with
Bragg's Law.

15. A method for reflecting light from a liquid crystal optical device,
comprising: applying a voltage to a first electrode and to a second
electrode, wherein a layer of cholesteric liquid crystal is disposed
between the first electrode and the second electrode, wherein the
cholesteric liquid crystal is arranged in domains, and wherein a
wavefront of a light wave having a wavelength within a range of
wavelengths changes after reflecting from the layer of cholesteric liquid
crystal; and varying the voltage to modify the amount of change of the
wavefront of the light wave after reflecting from the layer of
cholesteric liquid crystal.

16. The method according to claim 15, wherein at least one of the first
electrode and the second electrode is transparent, and the layer of
cholesteric liquid crystal changes state based on varying the voltage.

17. The method according to claim 15, wherein the liquid crystal optical
device comprises two layers of cholesteric liquid crystal, wherein the
two layers of cholesteric liquid crystal have the same handedness or
opposite handedness.

18. The method according to claim 17, wherein the liquid crystal optical
device comprises a layer of birefringent material disposed between the
two layers of cholesteric liquid crystal.

19. The method according to claim 17, wherein the first electrode is
coupled to a first substrate layer that is substantially flat, and the
second electrode is coupled to a second substrate layer that is
substantially flat.

20. The method according to claim 15, wherein the helical axes of the
domains have a plurality of orientations relative to an orientation of
the first and second electrodes.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims the benefit of U.S. Provisional
Patent Application No. 61/457,425, filed Mar. 24, 2011, which is
incorporated by reference.

[0003] Lenses are commonly used in optical systems to direct and/or
reconfigure light. In data communication systems, for example, lenses are
used to direct and/or reconfigure light provided by a light source to a
detector, optical fiber, or some other destination.

[0004] Lenses can be configured to act in a transmissive manner to allow
at least a portion of the light that is output from the light source to
pass through the lenses. However, another portion of the light from the
light source can be reflected from the lenses. In some implementations,
the reflected light can be directed to a back monitor photo detector that
is configured to detect the amount of reflected light. The back monitor
photo detector is configured to generate a signal corresponding to the
amount of reflected light. The signal can be provided to a controller
that adjusts the power of the light source to a desired power level.
Typically, the desired power level is a constant power level. Maintaining
a constant power level can be beneficial since some electrical and/or
optical parameters of some light sources, such as lasers, can vary due to
effects such as manufacturing tolerance, temperature, and aging. As such,
control of the power level of the light source can enhance the
performance of systems that use these light sources.

[0005] Various conventional techniques have been used to reflect light
from lenses, such as, for example, to a back monitor photo detector. In
one conventional approach, a tilted window (separate from the lens) is
provided above the back monitor photo detector and the light source,
where the tilted window includes a partially reflective coating. The
tilted window reflects a portion of the light beam from the light source
to the back monitor photo detector. Accordingly, in such an
implementation, both a partially reflective window and a separate lens
are provided in the path of the light beam. Having to manufacture and
mount both of these separate components can increase the cost of the
system. In addition, in some applications, there is insufficient room
between the light source and the desired destination to accommodate both
a partially reflective window and a separate lens.

[0006] Another conventional approach is to provide a concave lens that
includes a transmissive part for passing a portion of an incident light
beam and a reflective part for reflecting a portion of the incident light
beam. The reflective part is preferably substantially non-transmissive.
However, implementing a concave lens has various drawbacks, such as
putting restrictions on the size and shape of lens that can be used for
various applications. Therefore, concave lenses cannot be used in certain
applications.

[0007] Accordingly, what is needed in the art is a reflective lens that
overcomes drawbacks of conventional lenses discussed above.

SUMMARY

[0008] One embodiment provides a liquid crystal optical device. The liquid
crystal optical device includes a first substrate layer that is
substantially flat; a second substrate layer that is substantially flat
and parallel to the first substrate layer; and a layer of cholesteric
liquid crystal disposed between the first substrate layer and the second
substrate layer, where the layer of cholesteric liquid crystal is
arranged in domains, each domain having a helical axis, and wherein the
helical axes of the domains have a plurality of orientations relative to
an orientation of the first and second substrate layers, where a
wavefront of a light wave having a wavelength within a range of
wavelengths changes after reflecting from the layer of cholesteric liquid
crystal.

[0009] Another embodiment provides a method for reflecting light from a
liquid crystal optical device. The method includes applying a voltage to
a first electrode and to a second electrode, where a layer of cholesteric
liquid crystal is disposed between the first electrode and the second
electrode, where the cholesteric liquid crystal is arranged in domains,
and where a wavefront of a light wave having a wavelength within a range
of wavelengths changes after reflecting from the layer of cholesteric
liquid crystal; and varying the voltage to modify the amount of change of
the wavefront of the light wave after reflecting from the layer of
cholesteric liquid crystal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 is a schematic view of a cholesteric liquid crystal
structure in a planar state, according to one embodiment.

[0011]FIG. 2 is a schematic view of a cholesteric liquid crystal
structure in a focal conic state, according to one embodiment.

[0012] FIG. 3 is a schematic view of a cholesteric liquid crystal
structure in a homeotropic state, according to one embodiment.

[0013]FIG. 4 is a schematic view of a light beam reflected from a
cholesteric liquid crystal structure in a planar state, according to one
embodiment.

[0014] FIG. 5 is a schematic view of a light beam reflected from a
cholesteric liquid crystal structure in a planar state with inclined
domains of the cholesteric liquid crystal, according to one embodiment.

[0015] FIG. 6 is a schematic view of light beams reflected from a
cholesteric liquid crystal structure in a planar state with inclined
domains of the cholesteric liquid crystal having a distribution of
orientations, according to one embodiment.

[0016] FIG. 7 is a schematic view of light beams reflected from a
cholesteric liquid crystal structure in a planar state with inclined
domains of the cholesteric liquid crystal having a distribution of
orientations, according to another embodiment.

[0017]FIG. 8 is a graph illustrating reflectance of a cholesteric liquid
crystal structure relative to wavelength of incident light, according to
one embodiment.

[0018] FIG. 9 is a schematic view of a cholesteric liquid crystal
structure that includes two layers of cholesteric liquid crystal with
opposite handedness, according to one embodiment.

[0019] FIG. 10 is a schematic view of a cholesteric liquid crystal
structure that includes two layers of cholesteric liquid crystal with the
same handedness, according to one embodiment.

[0020] FIG. 11 is a graph illustrating reflectance of a cholesteric liquid
crystal structure that includes two layers of cholesteric liquid crystal
versus wavelength of incident light, according to one embodiment.

[0021] FIG. 12 is a schematic view of light beams reflected from a
cholesteric liquid crystal structure including two electrodes, according
to one embodiment.

[0022] FIG. 13 is a graph illustrating reflectance of a cholesteric liquid
crystal structure versus phase retardation of a birefringent plate,
according to one embodiment.

DETAILED DESCRIPTION

[0023] Embodiments of the invention provide a liquid crystal optical
device that includes at least one layer of cholesteric liquid crystal.
The cholesteric liquid crystal is arranged in domains of helicoidal
structures twisted around helical axes. The helical axes of the domains
have different orientations such that incident light waves having a
wavelength within a certain range, when reflected from the liquid crystal
optical device, change the forms of the wavefronts of the incident light
waves. Accordingly, the incident light waves can be focused onto a
particular target. An example of such a target is a back monitor photo
detector used to control the power output level of a light source.

[0024] Liquid crystal layers are commonly used in optoelectronic devices,
such as electronic displays, lenses with a tunable focal distance,
shutters, and modulators, among others. The liquid crystal in such
optoelectronic devices can be categorized into different phases
associated with the type of ordering of the liquid crystal. Examples of
different phases include the nematic phase, smectic phases, and chiral
phases. In the nematic phase, the liquid crystals have no positional
order, but they self-align to have long-range directional order with
their long axes roughly parallel. In the smectic phase, the liquid
crystals form well-defined layers and are, thus, positionally ordered
along one direction. In the chiral phases, the liquid crystals exhibit
handedness, characterized by layers of molecules, where the molecules in
one layer are rotated relative the molecules in the adjacent layers.

[0025] In electronic displays, the liquid crystals are typically in the
nematic phase. Lenses are also typically composed of nematic liquid
crystals. Nematic lenses have a non-uniform distribution of refractive
index due to the non-uniform distribution of orientations of the liquid
crystal molecules. Nematic lenses are transmissive. Smectic liquid
crystals are primarily used in modulators and shutters.

[0026] Embodiments of the invention provide a cholesteric liquid crystal
structure. The cholesteric liquid crystal structure described herein can
have three different states: a planar state, a focal conic state, and a
homeotropic state.

[0027] FIG. 1 is a schematic view of a cholesteric liquid crystal
structure 110 in a planar state, according to one embodiment. As shown,
the cholesteric liquid crystal structure 110 includes a substrate 102, a
substrate 103, and molecules of cholesteric liquid crystal 100. The
molecules of the cholesteric liquid crystal 100 are organized into
domains, where each domain is a helicoidal structure twisted around a
helical axis 101, known as an optical axis. The cholesteric liquid
crystal 100 is placed between two substrates 102 and 103. In one
embodiment, substrate 102 is transparent. The cholesteric liquid crystal
structure 110 in the planar state is periodic and reflects incoming light
according to Bragg's Law. Bragg's Law is a well-know principle of physics
that defines the angles of coherent and incoherent scattering from a
crystal lattice.

[0028]FIG. 2 is a schematic view of a cholesteric liquid crystal
structure 210 in a focal conic state, according to one embodiment. As
shown, the cholesteric liquid crystal structure 210 includes substrate
102, substrate 103, and molecules of cholesteric liquid crystal 200. In
the focal conic state, the cholesteric liquid crystal 200 does not form
uniformly orientated full domains, such as in the planar state. When
incident light interacts with the cholesteric liquid crystal 200 in the
focal conic state, a portion of the light is transmitted and a portion of
the light is scattered diffusely. A ratio between the portion of the
light transmitted and the portion of the light scattered diffusely
depends on the thickness of the layer of cholesteric liquid crystal 200
between the substrates 102, 103. If the thickness of the layer of the
cholesteric liquid crystal 200 is less than 10 μm, then the light is
primarily transmitted through the cholesteric liquid crystal 200 and the
layer is substantially transparent.

[0029] FIG. 3 is a schematic view of a cholesteric liquid crystal
structure 310 in a homeotropic state, according to one embodiment. As
shown, the cholesteric liquid crystal structure 310 includes substrate
102, substrate 103, and molecules of cholesteric liquid crystal 300. In
the homeotropic state, the cholesteric liquid crystal 300 aligns in
layers, oriented perpendicular to the substrates 102, 103. In one
embodiment, the cholesteric liquid crystal 300 arranges itself in the
homeotropic state when an external electric or magnetic field is applied
to the cholesteric liquid crystal structure 310. The cholesteric liquid
crystal structure 310 is substantially transparent in the homeotropic
state.

[0030] According to various embodiments, planar and focal conic states are
stable at zero voltage. Transition between theses states occurs after an
electrical or magnetic impulse is applied. For example, when an
electrical impulse of high voltage is applied, the liquid crystal
structure transforms to the homeotropic state. After turning off the
voltage, the liquid crystal transitions to the planar state. If a low
intensity electrical impulse is applied, then the liquid crystal
structure transitions to the focal conic state.

[0031]FIG. 4 is a schematic view of a light beam reflected from a
cholesteric liquid crystal structure 110 in a planar state, according to
one embodiment. In one embodiment, the cholesteric liquid crystal
structure 110 in FIG. 4 is the same as the cholesteric liquid crystal
structure 110 illustrated in FIG. 1. A light beam 401 is incident upon
the cholesteric liquid crystal structure 110 at an angle of incidence
α, defined by the angle between the light beam 401 and normal axis
400. The light beam 401 is reflected from the cholesteric liquid crystal
structure 110 (i.e., the illustrated reflectance beam 402) at an angle of
reflectance β, defined by the angle between the reflectance beam 402
and the axis 400. When the helical axes 101 of the domains of cholesteric
liquid crystal 100 are perpendicular to the substrates 102 and 103, as
shown in FIG. 4, the incident light beam 401 is reflected in a
mirror-like manner. Accordingly, the angle of reflectance β, in this
case, is equal to the angle of incidence α.

[0032] FIG. 5 is a schematic view of a light beam reflected from a
cholesteric liquid crystal structure 510 in a planar state with inclined
domains of the cholesteric liquid crystal, according to one embodiment.
As shown, the cholesteric liquid crystal structure 510 includes substrate
102, substrate 103, and molecules of cholesteric liquid crystal 500. When
the helical axes 504 of the cholesteric liquid crystal 500 are inclined
relative to the substrates 102 and 103, each helical axis aligned at the
same angle (i.e., angle γ relative to normal axis 505), the
incident light beam 501 is not reflected mirror-like to form reflectance
beam 502. In this case, the angle of reflectance β is not equal to
the angle of incidence α and depends on incline of the helical axis
501. The relationship between the angle of incidence α, the angle
of reflectance β, and the angle γ describing inclination of
the helical axis is:

where n is the average refractive index of the cholesteric liquid crystal
500.

[0033] FIG. 6 is a schematic view of light beams reflected from a
cholesteric liquid crystal structure 610 in a planar state with inclined
domains of the cholesteric liquid crystal having a distribution of
orientations, according to one embodiment. As shown, the cholesteric
liquid crystal structure 610 includes substrate 102, substrate 103, and
molecules of cholesteric liquid crystal 600. As also shown, the helical
axes 605-1 to 605-7 of the domains of cholesteric liquid crystal 600 do
not have uniform orientation relative to the orientation of the
substrates 102 and 103; rather, the domains of cholesteric liquid crystal
600 have a plurality of orientations relative to the orientation of the
substrates 102, 103.

[0034] As a result, for each of a plurality of incident light beams 601
and corresponding reflectance beams 602, the angle of incidence is not
equal to the angle of reflectance. The incident light beams 601 are
associated with a wavefront 603 identified in FIG. 6 with hash marks
between the incident light beams 601. A wavefront is the curve or surface
that includes the points in space reached by a wave or vibration at the
same instant in time as the wave travels through a medium. Also, the
reflectance beams 602 are associated with a wavefront 604 identified with
hash marks between the reflectance beams 602. As shown, the wavefront 604
of the reflectance beams 602 is different from the wavefront 603 of the
incident light beams 601.

[0035] As shown in FIG. 6, the reflectance beams 602 converge on one
another. A lens that creates reflectance beams 602 that converge on one
another is referred to as a "positive" lens, i.e., a lens with positive
focal length. The ability to focus the reflected light, such as with a
positive lens, can be useful when the reflected light is used as part of
a feedback mechanism, such as when the reflected light is directed to a
back monitor photo detector configured to measure the amount of reflected
light. A controller, which is coupled to the back monitor photo detector
and a light source, receives a signal from the back monitor photo
detector. The signal is indicative of the amount of reflected light
detected by the back monitor photo detector. The controller provides a
control signal to the light source that adjusts the power of the light
source such that the power of the light beam from the light source is
substantially constant. Using a lens that reflects light in a focused
manner, such as with a positive lens, allows for using a smaller photo
detector, which may reduce the overall cost of the optical system.

[0036] FIG. 7 is a schematic view of light beams reflected from a
cholesteric liquid crystal structure 710 in a planar state with inclined
domains of cholesteric liquid crystal having a distribution of
orientations, according to another embodiment. As shown, the cholesteric
liquid crystal structure 710 includes substrate 102, substrate 103, and
molecules of cholesteric liquid crystal 700. As also shown, the
orientations of helical axes 705-1 to 705-7 of the cholesteric liquid
crystal 700 are not uniform relative to the orientation of the substrates
102 and 103.

[0037] As a result, for each of a plurality of incident light beams 701
and corresponding reflectance beams 702, the angle of incidence is not
equal to the angle of reflectance. The incident light beams 701 are
associated with a wavefront 703 identified with hash marks between the
incident light beams 701. Also, the reflectance beams 702 are associated
with a wavefront 704 identified with hash marks between the reflectance
beams 702. As shown, the wavefront 704 of the reflectance beams 702 is
different from the wavefront 703 of the incident light beams 701.

[0038] As also shown in FIG. 7, the reflectance beams 702 diverge from one
another. A lens that creates reflectance beams 702 that diverge from one
another is referred to as a "negative" lens, i.e., a lens with negative
focal length.

[0039]FIG. 8 is a graph illustrating reflectance of a cholesteric liquid
crystal structure relative to wavelength of incident light, according to
one embodiment. The cholesteric liquid crystal reflects light of certain
wavelengths better than light of other wavelengths. In one embodiment,
the cholesteric liquid crystal structure best reflects light having a
wavelength between λ1 and λ2, where:

λ1=P {square root over (no2-sin2α)}
(Equation 2), and

λ2=P {square root over (ne2-sin2α)}
(Equation 3)

P is the pitch length of the chiral periodical structure of the
cholesteric liquid crystal in planar state, ne is the extraordinary
refractive index of the locally uniaxial structure, no is the
ordinary refractive index of the locally uniaxial structure, and α
is the angle of incidence. In the case of normal incidence (i.e.,
α=0), the cholesteric liquid crystal having right-handed helicoidal
structure reflects right-handed circularly-polarized light wavelengths
that lie between Pno and Pne; whereas, the cholesteric liquid
crystal having left-handed helicoidal structure reflects left-handed
circularly-polarized light with said wavelengths.

[0040] When the light refracted in the cholesteric liquid crystal does not
propagate along the helical axis, the state of polarization of the
selectively reflected light is elliptical. The ellipticity of the
reflected light depends on the angle between the direction of light
propagation inside the cholesteric liquid crystal layer and the helical
axis. After interaction of unpolarized light with cholesteric liquid
crystal, around 50% of the light energy is reflected within the range
λ1 and λ2, and around 50% of the light energy is
transmitted. This property is shown in FIG. 8 by the reflectance value of
approximately 0.5 within the range of wavelengths λ1 and
λ2.

[0041] FIG. 9 is a schematic view of a cholesteric liquid crystal
structure 910 that includes two layers of cholesteric liquid crystal with
opposite handedness, according to one embodiment. As shown, the
cholesteric liquid crystal structure 910 includes substrates 901, 902,
903, a first layer 904 of molecules of cholesteric liquid crystal, and a
second layer 905 of molecules of cholesteric liquid crystal. As also
shown, the orientation of the helical axes of the domains of cholesteric
liquid crystal in each layer are not uniform relative to the orientation
of the substrates 901, 902, 903.

[0042] As a result, for each of a plurality of incident light beams 906
and corresponding reflectance beams 907, the angle of incidence is not
equal to the angle of reflectance. The incident light beams 906 are
associated with a wavefront 908 identified with hash marks between the
incident light beams 906. Also, the reflectance beams 907 are associated
with a wavefront 909 identified with hash marks between the reflectance
beams 907. As shown, the wavefront 909 of the reflectance beams 907 is
different from the wavefront 908 of the incident light beams 906.

[0043] In the example shown in FIG. 9, the cholesteric liquid crystal in
the first layer 904 and the cholesteric liquid crystal in the second
layer 905 have the same pitch, but opposite handedness of the helicoidal
structure. Polarization of the incident light is expressed as a sum of
two circular polarizations with opposite handedness. For a
circularly-polarized light that has a polarization that coincides with
the handedness of the cholesteric liquid crystal 904 reflects from the
cholesteric liquid crystal 904. Light with orthogonal polarization (i.e.,
opposite handedness) passes through the cholesteric liquid crystal 904,
but reflects from the cholesteric liquid crystal 905. After reflecting
from the cholesteric liquid crystal 905, the light passes through the
cholesteric liquid crystal 904 again and, together with the light having
opposite polarization that initially reflected from the liquid crystal
904, forms the outgoing reflectance beams 907. The cholesteric liquid
crystal structure 910 that includes these two layers of the cholesteric
liquid crystal reflects 100% of natural light; whereas, a cholesteric
liquid crystal structure that includes one layer of cholesteric liquid
crystal reflects 50% of natural light.

[0044] FIG. 10 is a schematic view of a cholesteric liquid crystal
structure 1010 that includes two layers of cholesteric liquid crystal
with the same handedness, according to one embodiment. As shown, the
cholesteric liquid crystal structure 1010 includes substrates 1001, 1002,
1003, a first layer 1004 of molecules of cholesteric liquid crystal, and
a second layer 1005 of molecules of cholesteric liquid crystal. As also
shown, the orientation of the helical axes of the domains of cholesteric
liquid crystal in each layer are not uniform relative to the orientation
of the substrates 1001, 1002, 1003.

[0045] As a result, for each of a plurality of incident light beams 1006
and corresponding reflectance beams 1007, the angle of incidence is not
equal to the angle of reflectance. The incident light beams 1006 are
associated with a wavefront 1008 identified with hash marks between the
incident light beams 1006. Also, the reflectance beams 1007 are
associated with a wavefront 1009 identified with hash marks between the
reflectance beams 1007. As shown, the wavefront 1009 of the reflectance
beams 1007 is different from the wavefront 1008 of the incident light
beams 1006.

[0046] In the example shown in FIG. 10, the cholesteric liquid crystal in
the first layer 1004 and the cholesteric liquid crystal in the second
layer 1005 have the same pitch and the same handedness. In one
embodiment, the substrate 1002 comprises a birefringent plate 1002. In
cholesteric liquid crystal structures that include two layers of
cholesteric liquid crystals that have the same pitch and the same
handedness, at wavelengths in the range of reflection between
λ1 and λ2, the substrate 1002 acting as a
birefringent plate provides a phase retardation multiple 7C. Polarization
of the incident light is expressed as a sum of two circular polarizations
with opposite handedness. A circularly-polarized light having a
polarization that coincides with the handedness of the cholesteric liquid
crystal 1004 reflects from the layer of cholesteric liquid crystal 1004.
The light with orthogonal polarization (i.e., opposite handedness) passes
through the cholesteric liquid crystal 1004 and the birefringent plate
1002. The birefringent plate 1002 changes the polarization of the light
that passes through it to light of orthogonal polarization (i.e.,
opposite handedness). After passing through the birefringent plate 1002,
the light reflects from the second layer 1005 of cholesteric liquid
crystal. A cholesteric liquid crystal structure 1010 that includes these
two layers of the cholesteric liquid crystals reflects 100% of natural
light; whereas, a cholesteric liquid crystal structure that includes one
layer of the cholesteric liquid crystal reflects 50% of natural light.

[0047] FIG. 11 is a graph illustrating reflectance of a cholesteric liquid
crystal structure that includes two layers of cholesteric liquid crystal
versus wavelength of incident light, according to one embodiment. As
shown, at wavelengths in the range of reflection between λ1
and λ2, the reflectance is close to 1.0 (i.e., close to 100%
reflectance). The graph shown in FIG. 11 corresponds to the reflectance
of the cholesteric liquid crystal structure when both of the layers of
cholesteric liquid crystal have the same handedness, but are separated by
the birefringent plate as shown in FIG. 10, or when the layers of
cholesteric liquid crystal have the opposite handedness, as shown in FIG.
9.

[0048] FIG. 12 is a schematic view of light beams reflected from a
cholesteric liquid crystal structure 1210 including two electrodes 1204,
1205, according to one embodiment. As shown, the cholesteric liquid
crystal structure 1210 includes substrate 1201, substrate 1202,
electrodes 1204, 1205, and molecules of cholesteric liquid crystal 1203.
The electrode 1204 is positioned on an inside face of the substrate 1201,
and the electrode 1205 is positioned on an inside face of the substrate
1202. The cholesteric liquid crystal 1203 is positioned between the
inside faces of the substrates 1201, 1202. An electrical or magnetic
impulse (shown in FIG. 12 by the symbol V) can be applied across the
electrodes 1204, 1205.

[0049] According to some embodiments, the cholesteric liquid crystal 1203
changes its state (i.e., between planar state and focal conic state)
depending on parameters of the electrical or magnetic impulse applied
across the electrodes 1204, 1205. In one embodiment, the cholesteric
liquid crystal 1203 is in the planar state when no impulse is applied
(e.g., as shown in FIG. 1), the cholesteric liquid crystal 1203 is in the
homeotropic state when a high intensity impulse is applied (e.g., as
shown in FIG. 3), and the cholesteric liquid crystal 1203 is in the focal
conic state when an intermediate intensity impulse is applied (e.g., as
shown in FIG. 2). Parameters of the impulse (or a sequence of impulses)
that transform the cholesteric liquid crystal from one state to another
depend on the principal physical constants of the cholesteric liquid
crystal and the thickness of the cholesteric liquid crystal. For example,
there are cholesteric liquid crystals that transition to homeotropic
state when electrical field reaches intensity 5V/μm. After turning off
this electrical field, the cholesteric liquid crystal transition to
planar state. However, in one example, if an electric field of intensity
3V/μm is applied, which is not sufficient to transform the cholesteric
liquid crystal to homeotropic state, then the cholesteric liquid crystal
has focal conic state and will maintain the focal conic state after
turning off the electric field.

[0050] FIG. 13 is a graph illustrating reflectance of the cholesteric
liquid crystal structure shown in FIG. 10 versus phase retardation of the
birefringent plate 1002, according to one embodiment. This graph shows
that the retardation of the birefringent plate 1002 is not required to be
strictly at 180° (π) to obtain reflection close to 100%.

[0051] In sum, embodiments of the invention provide a liquid crystal
optical device that includes at least one layer of cholesteric liquid
crystal. The liquid crystal is arranged in domains having helical axes.
The helical axes of the domains have a plurality of orientations such
that incident light waves having a wavelength within a certain range,
when reflected from the liquid crystal optical device, change their
wavefront. Accordingly, the incident light waves can be focused onto a
particular target.

[0052] One aspect of the disclosed optical devices is that the substrates
that make up the optical device are flat, and not concave or convex.
These optical devices can therefore be suitable for many applications
that do not allow for concave or convex lenses.

[0053] Another aspect of the disclosed optical devices is that is that the
optical devices are reflective in a certain spectral range and
transmissive for the rest of the spectrum. Accordingly, the lenses do not
absorb light and maintain high optical power. Also, according to various
embodiments, the spectral range that is reflective is material-dependent
and can be adjusted according to the specific application.

[0054] Yet another aspect of the disclosed optical devices is that they
are electrically controllable. By adjusting the electrical or magnetic
impulse applied to the optical device, the reflectivity can be changed,
according to the desired properties of a particular application. For
example, in a 3D (three-dimensional) television application, the
electrical or magnetic impulse applied to the optical device can be
switched ON and OFF to switch the television from operating in a 3D mode
to operating in a 2D (two-dimensional) mode.

[0055] Embodiments disclosed herein may be used in a wide variety of
applications including telecommunications, computer, control, sensor,
manufacturing, solar cells and solar cell concentrators, and/or any other
suitable application.

[0056] Embodiments of the invention have been described above with
reference to specific embodiments. Persons skilled in the art, however,
will understand that various modifications and changes may be made
thereto without departing from the broader spirit and scope of
embodiments of the invention, as set forth in the appended claims. The
foregoing description and drawings are, accordingly, to be regarded in an
illustrative rather than a restrictive sense.

Patent applications by Hoi Sing Kwok, Hong Kong CN

Patent applications by Vladimir Chigrinov, Hong Kong CN

Patent applications by THE HONG KONG UNIVERSITY OF SCIENCE AND TECHNOLOGY